The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
The final drive assembly of the majority of passenger cars, sport utility vehicles and trucks includes a differential having two opposed pairs of bevel gears disposed in a cage which is secured to and rotates with a gear driven by the output of the transmission or transfer case. One pair of the gears are idler gears which each engage both of the other pair of gears which are coupled to and drive the axles.
While the primary purpose of the differential is to allow side-to-side variation in the rotational speed of the axles during cornering, a hypoid gear set which drives the differential typically provides an approximately three-to-one to four-to-one speed reduction and corresponding torque increase. So located, components of the transmission and propshaft may be somewhat less robust as they transmit lower torque levels than they would if such final drive speed reduction and torque multiplication did not occur proximate the axles.
Clearly, therefore, greater speed reduction and torque multiplication in the final drive assembly can achieve weight savings by permitting smaller and thus lighter weight components in a vehicle powertrain. However, as in all things engineering, there are limits to this approach. To achieve significant speed reductions and torque multiplications beyond that achieved by a typical hypoid gear set, an additional gear set will be required. This additional gear set introduces an energy loss and reduction in efficiency during all operating conditions. Furthermore, while the torque multiplication is desirable at lower speeds, it becomes less so at higher speeds. In fact, during many higher speed operating conditions, it would be desirable to directly drive the differential assembly and eliminate the speed reduction and energy loss of the final drive gear set.
In heavy duty trucks having eight, ten and twelve speed transmissions, these choices of gears are often augmented by two speed rear axles (differentials) which effectively double the choice of gears. Unfortunately, the cost and complexity of such devices renders them unsuitable for use or adaptation in lighter weight and duty vehicles such as passenger cars, sport utility vehicles and consumer trucks. For example, a two speed truck axle will have one and more typically two clutches which selectively couple an input to one of two gear assemblies or components that provide the two drive ratios to the rear differential.